While I'm waiting to hear back from Support, I'd thought I start asking the community...

I was migrating our Dev server from 9.0.2 to 9.0.6 when towards the end of the upgrade, I was at Initializing Tableau Server API Server when I received an initialization failure. Viewing the tabadmin log...

Checked the QA instance, where Guest is also Active and appears in the Default Site and the QA site. When you select Guest, the only Action you can perform is Group Membership. But to answer your question, I don't know about Dev.

The issue was with the Guest account being deleted in the Default site but available in the other site(s). It is unknown how the account was deleted, you can't do it manually. I do know that initially I had the Guest account disabled but later enabled. Regardless, a tableau engineer had to write an insert statement for the postgres database, used pgAdmin 3 to run the script. Once the Guest account was recreated in the default site, we were able to successfully upgrade.